Alzheimer's disease (AD) patients exhibit a variety of behavioral alterations including agitation, apathy, depression, anxiety, delusions, irritability and disinhibition. Most patients with AD exhibit neuropsychiatric symptoms, and behavioral changes become more frequent with advancing disease severity. The NPI is a valid and reliable means of assessing neuropsychiatric symptoms in patients with dementia. The NPI correlates with increasing disability in activities of daily living and increasing cognitive impairment. Physical illness contributes little to behavioral symptoms measured by the NPI. Reduced frontal lobe metabolism and perfusion have been identified in patients with apathy, agitation, psychosis and depression. Patients with elevated agitation scores on the NPI have a higher burden of frontal lobe neurofibrillary tangles than patients without agitation. The NPI is sensitive to behavioral improvements following treatment with cholinesterase inhibitors and psychotropic agents. Neuropsychiatric symptom profiles differ among dementia syndromes, and the NPI provides a means of assessing neuropsychiatric symptoms that may aid in differential diagnosis. Evaluation of neuropsychiatric symptoms is a critical aspect of dementia diagnosis and management.